AIB3 IIC2 IIIQ6 VIXVII: A Thioborate Halide Family for Developing Wide Bandgap Infrared Nonlinear Materials by Coupling Planar [BS3] and Polycations

Abstract

Developing high-performance infrared (IR) nonlinear optical (NLO) materials is urgent but challenging due to the competition between NLO coefficient and bandgap in one compound. Herein, by coupling NLO-active [BS₃] planar units and halide-centered polycations, six new metal thioborate halides ABa₃B₂S₆X (A = Rb, Cs; X = Cl, Br, I) composed of zero-dimensional [XBa_mRb_n/Cs_n] polycations and [BS₃] units, belonging to a new $A^I{B}_3^{\mathrm{II}}{C}_2^{\mathrm{III}}{Q}_6^{\mathrm{VI}}{X}^{\mathrm{VII}}$ family, are rationally designed and fabricated. The compounds show an interesting structural transition from Pbcn (ABa₃B₂S₆Cl) to Cmc2₁ (ABa₃B₂S₆Br and ABa₃B₂S₆I) driven by the clamping effect of polycationic frameworks. ABa₃B₂S₆Br and ABa₃B₂S₆I are the first series metal thioborate halide IR NLO materials, and the introduction of [BS₃] unit effectively widens the bandgap of planar unit-constructed chalcogenides. ABa₃B₂S₆Br and ABa₃B₂S₆I, exhibiting wide bandgaps (3.55-3.60 eV), high laser-induced damage thresholds (≈ 6 × AgGaS₂), and strong SHG effects (0.5-0.6 × AgGaS₂) with phase-matching behaviors, are the promising IR NLO candidates for high-power laser applications. The results enrich the chemical and structural diversity of boron chemistry and give some insights into the design of new IR NLO materials with planar units.

Keywords: high LIDT; nonlinear optical material; planar unit; structural transition; thioborate halide.